AMD、CMD、UMD、CommonJS

amd-cmd-umd-commonjs.md

Over the years there’s been a steadily increasing ecosystem of JavaScript components to choose from. The sheer amount of choices is fantastic, but this also infamously presents a difficulty when components are mixed-and-matched. And it doesn’t take too long for budding developers to find out that not all components are built to play nicely together.

To address these issues, the competing module specs AMD and CommonJS have appeared on the scene, allowing developers to write their code in an agreed-upon sandboxed and modularized way, so as not to “pollute the ecosystem”.

AMD（Asynchromous Module Definition)

Asynchronous Module Definition (AMD) has gained traction on the frontend, with RequireJS being the most popular implementation.

Here’s module foo with a single dependency on jquery:

//    filename: foo.js
define(['jquery'], function ($) {
    //    methods
    function myFunc(){};

    //    exposed public methods
    return myFunc;
});

And a little more complicated example with multiple dependencies and multiple exposed methods:

//    filename: foo.js
define(['jquery', 'underscore'], function ($, _) {
    //    methods
    function a(){};    //    private because it's not returned (see below)
    function b(){};    //    public because it's returned
    function c(){};    //    public because it's returned

    //    exposed public methods
    return {
        b: b,
        c: c
    }
});

CMD（Common Module Definition)

Standard locates at seajs/seajs#242. It keeps more compatibilities with CommonJS and Node.js Modules.

• Published by Chinese people who is developing SeaJS.
• It is like AMD.

define((require, exports, module) => {
  module.exports = {
    fun1: () => {
       var $ = require('jquery');
       return $('#test');
    } 
  };
});

CommonJS

CommonJS is a style you may be familiar with if you’re written anything in Node (which uses a slight variant). It’s also been gaining traction on the frontend with Browserify.

Using the same format as before, here’s what our foo module looks like in CommonJS:

//    filename: foo.js

//    dependencies
var $ = require('jquery');

//    methods
function myFunc(){};

//    exposed public method (single)
module.exports = myFunc;

And our more complicate example, with multiple dependencies and multiple exposed methods:

//    filename: foo.js
var $ = require('jquery');
var _ = require('underscore');

//    methods
function a(){};    //    private because it's omitted from module.exports (see below)
function b(){};    //    public because it's defined in module.exports
function c(){};    //    public because it's defined in module.exports

//    exposed public methods
module.exports = {
    b: b,
    c: c
};

UMD（Universal Module Definition)

Since CommonJS and AMD styles have both been equally popular, it seems there’s yet no consensus. This has brought about the push for a “universal” pattern that supports both styles, which brings us to none other than the Universal Module Definition.

The pattern is admittedly ugly, but is both AMD and CommonJS compatible, as well as supporting the old-style “global” variable definition:

(function (root, factory) {
    if (typeof define === 'function' && define.amd) {
        // AMD
        define(['jquery'], factory);
    } else if (typeof exports === 'object') {
        // Node, CommonJS-like
        module.exports = factory(require('jquery'));
    } else {
        // Browser globals (root is window)
        root.returnExports = factory(root.jQuery);
    }
}(this, function ($) {
    //    methods
    function myFunc(){};

    //    exposed public method
    return myFunc;
}));

And keeping in the same pattern as the above examples, the more complicated case with multiple dependencies and multiple exposed methods:

(function (root, factory) {
    if (typeof define === 'function' && define.amd) {
        // AMD
        define(['jquery', 'underscore'], factory);
    } else if (typeof exports === 'object') {
        // Node, CommonJS-like
        module.exports = factory(require('jquery'), require('underscore'));
    } else {
        // Browser globals (root is window)
        root.returnExports = factory(root.jQuery, root._);
    }
}(this, function ($, _) {
    //    methods
    function a(){};    //    private because it's not returned (see below)
    function b(){};    //    public because it's returned
    function c(){};    //    public because it's returned

    //    exposed public methods
    return {
        b: b,
        c: c
    }
}));